Process for making composite ferrites



Oct. 3, 1961- Filed May 1, 1956 I'WONT-RJ-BACKLOSS RATIO/N DEC/EELS LEGRAND G. VAN UITERT PROCESS FOR MAKING COMPOSITE FERRITES 2 Sheets-Sheet1 FIG.

FREQUENCY IN K/LOMEGACYCLES INVENTOR L. 6. V4 UITERT A Tram/Er v Oct. 3,1961 LE GRAND G. VAN UITERT PROCESS FOR MAKING COMPOSITE FERRITES FiledMay 1, 1956 FPONT- 7'O-BACK L055 EA 770 IN DEC/EELS N 2 Sheets-Sheet 2FREQUENCY /N K/LOMEGACVCLES INVENTOR ZZLMM 03C,

ATTORNFV United States Patent 3,062,929 PROCESS ran MAKINGCUMPOSITEFERRITES Le ,Grand G. Van Uitert, MorrisrTownship,-Morris(Younty, Ni, assignor to Bell Telephone Laboratories, Incorporated, NewYork, N.Y.,a corporationof New .York

Filed May 1, 1956, Ser. No. 581,859 1 Claim. (Cl. 252 62.5)

This invention relates to methods for the preparation of inhomogeneousstructures composed of magnetic ferrite netic, ferrite materials soproduced.

Ferrites are magnetic ceramic compositions of general formula MFe 0where M rnay bepnepnmorelof a materials, and to the inhomogeneousstructures of mtgvariety of divalent metal ions known in the art. Commondeviations from the composition generalized above include partialsubstitution of ferric ion with othertrivalent metal ions, orcompounding the ferrite to be iron deficient, that is, to show adeparture from'the stoichiometricamount of Microwaves whose frequenciescoincide with the responant frequency of a ferritebody'throughwhich'they' are passing are strongly absorbed by theferrite. One recent device which makes use of this phenomenon is thetraveling wave tube described by I. S. Cook, R. Kompfner and H. 'Suhl inan article in the Journal of Applied-Physics, volume 26,

pages 1180 through 1182, 'September1955. Other devices, such asresonance isolator devices,known in the art,'can also be mentioned asusing ferrite compositions 'to absorb microwave radiation.

For a number of device applications, including the traveling wave tubementioned above, and also in broadband resonance isolator device's, toname a second example, it is desirable to use a magnetic ferrite showingresonance absorption of microwaves over a broad band of microwavefrequencies. "Asany one ferrite" composition generally shows resonanceabsorption over a "comparatively smallband of frequencies surrounding aresonance absorption peak, the fulfillment of the concept of a'singlematerial showing bnoadbandabsorption has not'previously beenaccomplished.

Some work has been done in the prior art to produce ferrite structureswhich would'combine in one structure the desirable characteristics ofmore than one ferrite; The British patent to Philips ElectricalIndustries, Limited, No. 734,243, published July27, 1955 describesferromagnetic ferrite cores made up of assembled partialc-ores composedof different ferrite materials." Cores molded from layers of pulverizedferrites kept carefully separated prior to sintering are also described.

Whatever the success of these prior are composites may be for use'ascores in inductive devices, their use in many microwave devices wouldnotbe indicated. A ferrite composite whose composition changes grossly fromone portion of the structure to nother presehts assay of changingdielectric constant to anadva'ncing iemwavaror e51 am le. The velocityor the impinging wave could be jaltered in passing fromone region withinthe body to artother as a' result of this changing dielectric constant.Similarly, in such a'structure as the wave encounters different ferriteportions of different permeability, ana mtinuity in the magnetic fieldin the structure, disruptive of the wave path, may be met. For use atmicrowave fredis is a tru u wh c is omogen ou to the micro.-

' preferred 2 wave, though perhaps inhomogeneous when measured by otherstandards, 'is preferred. i

" 'To' form a composite structure which is homogeneous to'microwaves,discontinuities in composition should be no 'larger than about one-fifthof a wavelength and are preferably smaller. Thus for a wave of afrequency of 1000 megacycles, having a wavelength of only about 0.3centimeter, discontinuities in structure should be no greater than 0.06centimeter in size, preferably less. For a frequency of megacycles,discontinuities in a structure, through which the microwave is passing,greater than 0.6 centimeter should be avoided, and preferably, muchsmaller discontinuities should be present. As the frequen'cy of themicrowave decreases, the limitations on the minimum size of adiscontinuity decease inv stringenc i w Over the frequency band ofinterest in most microwave technology, said band ranging from about 100megacycles to about 100,000 megacycles, structures in which inhomo-'geneities range in size from about 0.003 centimeter-to about 0.5centimeter in size still present an essentially homogeneous structure tomany microwaves, especially those of lower frequency. As the frequencyof the micro-- as those taught in the aforementioned British patentspecification may pnove awkward. Thus machining interlocking parts of astructure from different materialsand fitting the parts into a wholewould become more complex as de-- sired structural shapes become moreintricate. Similarly, molding shapes from layered ferrites would be more.dif-- ficult,.if notimpossible, when structures more complicated thanthe simplest geometrical forms were desired. Even:

in molding simple forms having discretely layered per-- tions, further,a tendency of the structure to buckle orwarp on firing due tonon-uniform shrinkage of the different portions of the structure hasbeen encountered.

By the methods of the present invention, a single ferrite: structurehomogeneous to microwaves but with broadbandl resonance absorptioncharacteristics over a desired ire-- quency range can be quite easilyfashioned. The struc-- ture, which shows uniformity and homogeneity formicro;- wave purposes, is, however, inhomogeneous. Within the:structure, a regional variation in ferrite composition can befound: eachsuch subdivision contributes some of individual characteristics to thewhole. But by avoiding" gross variations in the ferrite composition ofthe structure as one passes from one gross portion of the structure toanother, the overall characteristics of the structure and material,including the permeability and dielectric constant, are kept uniform."'In particular, in preparing composite structures for resonanceabsorption of microwaves,the magnetic ferrites which are to be thecomponents in the overall structure are selected with reference to theirresonance characteristics. Materials showing resonance peaks at spacedintervals within a broad band of microwaves are included, and in thismanner a broadband resonance property is imparted to'the compositestructure produced. "i

' In the accompanying drawings:

FIG. 1 is a graph, the abscissa of which is frequency measured inkilomegacycles and the ordinate of which represents the front-to-backloss ratio in a ferrite structure measured in decibels, showing curvesfor five sep-- arate ferrites and for a composite material formed fromsaid ferrites by the methods of the present invention;

FIG. 2 is another graph of front-to back loss ratio in.

decibels versus frequency in kilomegacycles showing three absorptioncurves for three difi'erent composite ferrite structures;

FIG. 3 is a third graph of loss ratio in decibels versus frequency inkilomegacycles showing two curves which compare the experimentallymeasured loss characteristics of a composite ferrite with theoreticalloss characteristics predicted by an arithmetic summation of individuallosses in an equivalent quantity of the component ferrites; and

FIG. 4 is a schematic view of an enlarged section of the structure of acomposite body produced by the meth ods of the present invention.

In FIG. 1 curves 11 through 15 show graphically the resonance absorptioncharacteristics of a representative group of ferrites, plotted as afunction of frequency. As

can be seen on the drawing, the ferrite represented by curve 11 peaks atapproximately 1.7 kilomegacycles. fCurves 12, 13 and 14, eachrepresenting the resonance losses of a different ferrite composition,peak at approximately 3.1 kilomegacycles; 4.5 kilomegacycles, and 4.9kilomegacycles, respectively. Curve 15 represents only a portion of theresonance absorption curve of a fifth fer- :rite material. Theabsorption peak has not been reached jwithin the frequency range ofinterest.

Curve 16, which is of primary interest, represents the empiricalresonance absorption characteristics of a composite material compoundedof equal parts by weight of the five ferrites shown in curves 11 through15, prepared by the method disclosed herein. As can be seen from thegraph in FIG. 1, substantial resonance absorption has been imparted tothe composite structure over a frequency band between approximately 2kilomegacycles and 7 kilomegacycles, with maximum absorption being shownat about 4.2 kilomegacycles. The shape of the curve of the composite canbe varied, within limits, by variation in the relative proportions ofthe component ferrites present in the composite. The maximum of curve 16of FIG. 2 may be moved to regions of lower frequency, if desired, by anincrease in the relative amounts of the ferrites whose losses are shownas curves 11 and 12., for example, if such is desirable for a given useof the composite. Conversely, a shift to higher frequencies may beaccomplished by the inclusion of increased proportions of the ferritesrepresented in curves 14 and 15.

The curves 11 through 15 represent measurements on the following ferritecompositions, in order:

gr.n oas m or ti; gm .ts iu oa ne os os na or ti; o.as o.t5 1.9 on2 4i;OJl OA m om ti By the choice of a combination of magnetic oxidesdifferent from that illustrated in FIG. 1, a suitable composite may beobtained which shows resonance absorption in another portion of thefrequency spectrum altogether.

The loss ratios measured in FIG. 1 were for a unidirectional wavedirected along the longitudinal axis of a helix of the ferrite beingtested. A direct-current magnetic field directed axially along the helixgave a longitudinal field of between about 300 gausses and 600 gausses.A circumferential component of the field around the helix of the ferritewas responsible for interaction with the wave.

The effect of changes in the proportion or composition of the individualcompositions making up the final inhomogeneous structure is shown inFIG. 7.. Curve 16 of that figure represents the loss ratio dependence onfrequency of the composite shown also in FIG. 1. Curve 21 of FIG. 2shows the shift in maximum absorption to a frequency approximately 0.8kilomegacycle lower occasioned by a slight variation in the compositiongiving curve 16. The ferrite represented by curve 15 of FIG. 1 has beenchanged from 20 percent by weight of the material of curve 16 to only 10percent by weight of the material of curve 21, while 10 percent byweight of the ferrite showing a maximum absorption of 16 decibels at 1kilomegacycle', has been substituted. Curve 22 shows the characteristicsof a composite different entirely in com- The measurements presentedgraphically in FIG. 2 were madeunder the same conditions as thosedescribed for the samples of FIG. 1.

In FIG. 3, the empirically determined absorption characteristics of thecomposite represented by curve 16 in FIGS. 1 and 2 are compared withcurve 31, the weighted mathematical summation of curves 11 through 15 ofFIG. 1. The curves show the measured absorption characteristics of thecomposite and the calculated absorption values which would be obtainedif an equal quantity of effective absorbing material were to beassembled, for example in tandem, from the five separate ferrites makingup the composite. Under the experimental conditions, described for FIG.1, used in measuring the absorption indicated by curve 16, the length ofthe ferrite helix tested was critical in determining the amount ofabsorption of the helix at a given frequency. The curve 31 would then,for example, represent the absorption of a helix of equal length as thatmeasured for curve 16, but composed of five joined pieces, eachone-fifth of the total length, of the separate ferrites used inmanufacturing the composite measured in curve 15.

The good match of the calculated curve 31 with the empirical curve 16shows that the absorption of a proposed oomposite may be predicted withfair accuracy from a curve summing the loss curves of the componentmaterials proposed for the composite structure.

FIG. 4 is a representation in section of the structure of "a compositeferrite produced by the methods of the present invention magnified by afactor of about 50. In FIG. 4 the inhomogeneity of the structuredepicted is visually apparent. Some portions of the structure, forinstance areas 41 and 42, appear to have fairly clearly definedboundaries. Other distinguishably dissimilar portions, for example 43'and 44, different in appearance and also probably different incomposition, merge almost indistinguishably in a zone created bydiffusion during sintering. Very clearly bounded areas 45 are,apparently, voids in the fired structure. Portions 41, 42, 43 and 44have no reference to specific ferrite compositions, and arerepresentative of different portions of any of the fired inhomogeneouscompositions mentioned hereinafter.

Inhomogeneous structures of the kind here considered may be formed fromany combination of ferrites. Depending on the broadband characteristicssought, the nature, number, and proportions of the component materialscan be varied. The relatively good correlation between the empiricallymeasured curve of a composite and the weightedmathematical summation ofthe absorptions observed for the individual ferrites, as shown in FIG. 3for example, permits agood estimation of the absorption characteristicsof a proposed composite before experimental testing of the compoundedand fired product.

Among the ferrites that have proved useful in the manu facture of thebroadband composites are the following. In the formulas, uncertaintyconcerning the amount of oxygen bound with the metallic constituents isreflected by'the plus-minus notation on oxygen content. This uncertaintyarises from a lack of exact knowledge of the extent ofoxidation-.onreduction of the metallic materials, particularlyiron,after firing.

The list oit' ferrite materials presented is exemplary only, and is notto be construed as limiting combinations .possiblein composites formedby themethod described herein. As mentioned, anytferrites may becombined metals whose oxides make up the ferrite. These startingcompounds may be 1 oxides or compounds convertible to oxides, such ashydroxides or carbonates. V

Sintering converts compoundsto'oxides, if other than oxides are used asa'startingmateri'al', and reacts the oxides to form the ferrite.The-Jsintered material may be ground and resintered-one lor moreadditional times to insure uniformity in the final ferrite composition.After the last sintering step in. the preparation of each of theconstituents desired to be. formed into a'comp'osite, the

fired composition isground, conveniently by the usual ball-millingtechniques. a

:The millingmay be done dry, but is conveniently carried out in thepresence of water, acetone, alcohol, or

carbon tetrachloride A binder and lubricant-may be added during the:milling,.:polyvinyl alcohol or opal wax (hydrogenated. castor= oil)being preferred when ball-milling with'water. .1 Parafiin or Halowax(chlorinated. napththalene) are useful with non-aqueous solvents such ascarbon tetrachloride.

Each of the groundferritematerials is then dried by filtration andevaporation, for example, and thedried material isscreenedso-athat:aggregates predominantly ranging in particle size between about 0.5centimeter and about 0.01. centimeter in thelaigest dimension areobtained. Particles"abet-weenv about 0.25 centimeter and about 0.01centimeter in size are also conveniently used. During pressingandfiring, a compression and shrinkage of the aggregatesto aboutone-third of their origin-a1 volume is experienced. Thusyt'o obtaindiscontinuities between 0003 centimeter; .-and 0.2 centimeter in a firedcomposite,,.the aggregates before mixing are preferably roughly betweenabout 0.011 centimeter and about 0.5 centimeter in size, as mentioned,Crumbling the aggregates to sizes roughly between 0.25: centimeter and0.0, l centimeter will give discontinuities in the fired body between.about 0.1 centimeter and.'0.003 centimeter in size. A similar roughcorrespondence in the aggregate size to be employed and the' size 'ofthediscontinuities wanted in the; fired body .can be worked out in othercases. After screening as mentioned above, the individual ferriteaggregates areoombined, one with another, in the proportions desired inthe final composite. I 1 T This combination-is critical". 'Mixingof theindividual materials should not be so thorough that diffusion, possomesibly induced by the final firing of the pressed composite, would" bringabout homogenization or the body." Mixing-shouldbe" sufficientto-bring-about a' distribution of each of the individual ferritecomponents throughout the final mixture; yen-tin any portion'of thebody, discrete ample, for-e 30 second interval. Mixing, in any case,

should not be carried out for such time as will pulverize anysubstantial-portion of the ferrite aggregates tosizes below 0'.0 lcentimeter, as set forth above. 3 When aggregates ot-comparatively largesize are being mixed,-mixing can'be' more thorough than when smallerparticles are mixed."- For the larger particles, the size of theparticle itself assures some inhomogeneity to a given section ofmaterial, even if some diffusion occurs on firing. If small particlesare'toothoroughly mixed, firing, especially for long periods may causehomogenization of the structure through diffusion, with a resultant lossof the presence of the individual distinct components. The

use of aggregates much below 0.01 centimeter usually atfords too greatopportunity for homogenization during firing, especially where mixinghas been too thorough or firing too long, to make aggragates smallerthan 0.0 1 centimeter in size of much interest.

After mixing, the material may be formed by pressing. Forming isfacilitated by a binder optionally included in the component aggregates,as discussed Forming may be done withouta binder present also, withperhaps a small amount of moisturebein g'either retained in theaggregates before mixing or'bei'ng added to the mixed materials beforepressing- Forming' is usually done at pressures between 10,000 po 'nds'per square inch and 50,000 pounds per square inch.

If a binder has been incorporated into the pressed detail, the detail isconveniently dewaxed after pressing by heating in air. A convenientschedule for dewaxing comprises bringing the pressed parts to atemperature of 400 Clover a6 hour period and then maintaining a 400 (2.temperature over another o hour interval.

It is to be noted that'the function of the binders or lubricantspreviously mentioned herein is to facilitate the pressing" operation bygiving some coherence of the mixed particles one to another.Thelubiicant acts also to ease the flow of the solid material undercompression in the die during forming. Both binders and lubricants areoptionally includedfand are not relied upon in the invention in any wayto give a preferred arrangement of particles prior to pressing. Nopreformed layers, 'or any other ordered arrangement, is maintained bythe use of the binders and lubricants. A random mixing is the primerequirement of the invention, so that a product, homogeneous tomicrowaves, is obtained.

Final firing is carried'out' at temperatures and for times sufficienttosinter the pressed composites, but not long enough that diffusionsuflicient to homogenize the body can occur. Temperatures between 1000C. and 1400 C. are usual, with a temperature of 1250 C. being best formost firings. T

:The time for which the ceramic composites are fired varies betweenliminutesand 20 hours, depending on the composition" of the composites.If copper-containing components are present, for example, firing timesmay be short, as copper oxides have a high fluxing activity in thecomposition. On the other hand, aluminum oxide, for

= example, is diflicult'to sinter,and aluminum-bearing ferrites mayrequire heating for periods of time approaching the 20 hour figure. Inmost other cases, those not involving special'treatments, for example,due the copper oraluminum ferrites, a firing'time of 3 hours to 5 hours"is typical. The composite whose absorption is shown as curve 16 onFIGS. 1, 2 and 3, for example, was fired f 3 hours at 1250" C. a 1

Firing may be done in an atmosphere of air when firing he is working.

The preparation of a composite ferrite prepared by a preferred manner ofpracticing the invention is described below. It is to be understood thatthe example given is illustrative only, and is not to be construed aslimiting the scope of the invention in any manner.

Example 1 A ferrite of the composition 0.e 0.4 1 .9 o.o2 4

was prepared by mixing the following ingredients in the proportionsgiven:

Material: Parts by weight NiCO 7 1.1 ZnO 32.6 F3203 =MnCO 2.3

Mixing was done for 15 minutes in water in an Eppenbach mill. The mixedmaterial was filtered, dried in an oven at 110 C., and granulated usinga 20 mesh Standard screen with sieve openings of 0.84 millimeter. Thematerial was calcined at 900 C. for 16 hours, then ballmilled with waterovernight. After milling, the material was again filtered and dried inan oven at 110 C. The dried ferrite was again ball-milled, this time incarbon tetrachloride to which Halowax was added in an amout equal topercent by weight of the charge. Milling was continued for about 2hours, after which the solvent was evaporated from the wax-coatedparticles while stirring the particles. The resultant conglomerates werethen screened on a 60 mesh Standard screen with sieve openings of 0.25millimeter.

In identical fashion, two ferrites of the compositions noted wereseparately prepared by mixing the materials listed in the proportionsgiven and subjecting them to further treatment as noted above.

Two more ferrites were separately prepared as in the preceding examples,except that calcining was carried out at 1000 0., rather than 900 C.Their compositions are given below, with the ingredients used incompounding them.

gr.o o.15 1.a o.1 4=i= Material: Parts by weight M co 84.3 A1(OH) 11.7Fegog 127.6 MnCO 11.5

Material: Parts by weight MgCO 84.3 Al(OH) 27.3 Fe O 111.7 MnCO 11.5

After preparation of the five ferrites as outlined above, equal portionsby weight of each of the dry powdered aggregates were loosely mixed byhand for about a minute. The mixed aggregates were placed in a die blockand compressed under a pressure of about 20,000 pounds per square inch.The pressed composite was dewaxed by heating the material from roomtemperature up to 400 C. over a period of six hours, and maintaining thecomposite at that temperature for another 6 hour period.

The composite was then fired at 1250 C. for 10 hours in an atmosphere ofoxygen.

The broadband resonance properties of the fired composite are shown ascurve 16 of FIG. 1.

What is claimed is The method of preparing a composite ferrite bodyshowing broad-band resonance absorption for microwaves which comprisesseparately sintering five ferrite compositions which individually showresonance peaks at spaced intervals within the frequency spectrum forwhich broadband resonance is desired, the metallic constituents of saidcompositions being present in the following amounts, the non-metallicconstituent being oxygen:

powdering said sintered ferrite compositions, screening said powderedferrite compositions to obtain ferrite aggregates between 0.01centimeter and 0.5 centimeter in their longest dimension, loosely mixingin approximately equal parts by weight said different ferrite aggregatesbut retaining in said mixture aggregates 0.01 centimeter to 0.5centimeter in their longest dimension, shaping a body from the mixtureso prepared and firing said shaped body in an oxidizing atmosphere at atemperature between 1000 C. and 1400" C. for a time sutficient to sintersaid mixture without extensive homogenization of said mixture byinterdiifusion between said different ferrite regions, whereby acomposite structure, homogeneous in gross but rendered inhomogeneous bythe presence 'of a mixture of discrete regions composed essentially ofthe individual component ferrites is produced, said regions beingbetween about 0.003 centimeter and 0.5 centimeter in their largestdimension.

References Cited in the file of this patent UNITED STATES PATENTS2,700,023 Albers-Schonberg Jan. 18, 1955 2,73 6,708 Crowley Feb. 28,1956 2,762,777 Went et a1 Sept. 11, 1956 FOREIGN PATENTS 679,453 GreatBritain Sept. 17, 1952.

495,355 Belgium Oct. 26, 1950 524,097 Belgium Nov. 30, 1953 1,110,334France Oct. 12, 1955 1,116,092 France Jan. 23, 1956 1,116,093 FranceJan. 23, 1956 OTHER REFERENCES Kordes et a1.: Chemical Abstracts, vol.46, column 4411,

May 25, 1952.

Gorter: Philips Research Reports, vol. 9, No. 6 pp. 403-443, Dec. 1954.J. Inst. of Elect. Engineers, Japan, November 1937, pp. 5, 7.

Snoek: Physica III, No. 6, p. 481, June 1936.

